my $a if 0;
my $b = 3;

##</code><code>##

$a=1;
$b=2;
sub x {
    my (0?$a:$b) = shift;
    print"a=$a\nb=$b\n";
    $a=shift
}
x(3,30);
x(4,40);
__END__
# OUTPUT:
a=
b=3
a=30
b=4